1. Field of the Invention
The present invention relates to a photoelectric switch device for detecting an object in a specified region of detection.
2. Description of Related Art
Typically, various photoelectric switch devices are widely used to detect an object in a specified region of detection in, for example, a manufacturing line in a factory. Such a photoelectric switch device projects light rays toward a specific field of detection in which an object is expected to be present and detects an object in the specific region of detection on the basis of a value relating to light rays reflected by the object. A conventional triangulation method is used for some of this kind of photoelectric switch devices. The triangulation type photoelectric switch device is also referred to as a distance setting type of photoelectric switch device. Reference is made to FIGS. 25 through 27A to 27C which show the structure and operation of one of the conventional triangulation type photoelectric switch devices for the purpose of providing a brief background that will enhance an understanding of the present invention.
Referring to FIG. 25, a triangulation type photoelectric switch device 800 comprises a light emitting diode 801 as a light source, a projection lens 802, a focusing lens 803 and a photoelectric position sensing device (PSD) 804. The light emitting diode 801 is coaxially aligned with the optical axis Xp of projection lens 802. The projecting lens 802 and the focusing lens 803 are arranged with a specified separation in the same vertical plane including the optical axis Xp of the light projection lens 802. The light projecting lens 802 directs light rays emanating from the light emitting diode 801 toward an object 900 located remotely from the photoelectric switch device 800 in the optical axis Xp of the light projection lens 802. The light rays are then reflected back by the object 900 and focused on the photoelectric position sensing device 804 by the focusing lens 803 to form a light spot on the photoelectric position sensing device 804.
The position of the light spot Lp on the photoelectric position sensing device 804 varies according to the distance of the object 900 from the photoelectric switch device 800, and more particularly to the photoelectric position sensing device 804. This distance is hereafter referred as switch-to-object distance. Specifically, the light spot Lp is formed at one of opposite ends, for example, an end e1, of the photoelectric position sensing device 804 when the object 900 is at the closest switch-to-object distance of a specified field of detection D0 and shifts in position towards another end e2 of the photoelectric position sensing device 804 as the object 900 moves away from the photoelectric switch device 800, i.e. as the switch-to-object distance increases.
The photoelectric position sensing device 804 generates two position signals N and F having levels according to positions of the light spot Lp formed thereon. Specifically, the position signal N has a photoelectric signal (current) level proportional to the distance of the light spot Lp from the end e1 of the photoelectric position sensing device 804. The position signal F has a photoelectric signal (current) level proportional to the distance of the light spot Lp from the other end e2 of the photoelectric position sensing device 804. Accordingly, the switch-to-object distance of the object 900 is found on the basis of these two position signals N and F.
The photoelectric switch device 800 has a specified field of detection or axial detectable region of detection D0 that is defined between the closest axial position P1 and the remotest axial position P2 which are axial limit positions for light rays incident upon the photoelectric position sensing device 804 through the focusing lens 803. The axial detectable region of detection D0 is divided into two sub-regions on both sides of a specific axial position ST that is variably preset, namely a front half axial region D1 that is on a side of the specific axial position ST close to the photoelectric switch device 800 and is used as an effective detection region and a rear half axial region D2 that is on a side remote from the photoelectric switch device 800 with respect to the specific axial position ST and is excluded from detection as an ineffective detection region.
The photoelectric switch device 800 is configured so as to detect an object 900 in the axial detectable region D0. However, the photoelectric switch device 800 determines that the object 900 is present within the effective detection region D1 only when light rays incident upon the photoelectric position sensing device 804 are from the object 900 positioned in the effective detection region D1.
Referring to FIGS. 26A and 26B showing a method of determining whether an object 900 is present within the effective detection region D1 on the basis of position signals N and F from the photoelectric position sensing device 804, a difference in level between N and F position signals (N−F) represents an axial position of the object 900 within the axial detectable region D0 and can be used as a position signal. In practice, the position signal is expressed as a ratio of the position signal level difference (N−F) relative to the total level of N and F position signals (N+F) for the purpose of normalization. The normalized position signal {(N−F)/(N+F)} is examined with respect to a threshold value TH, which meets with the position signal relating to the specific axial position ST, to determine whether an object 900 is present within the effective detection region D1. That is, it is determined that an object 900 is present within the effective detection region D1 when the position signal {(N−F)/(N+F)} is greater than the threshold value TH as shown in FIG. 26A or that an object 900 is not present within the effective detection region. D1 but is within the ineffective detection region D2 when the position signal {(N−F)/(N+F)} is smaller than the threshold value TH as shown in FIG. 26B. The threshold value TH may be varied to shift the limit axial position ST so as to change, expand or narrow, the effective detection region D1 and the ineffective detection region D2.
FIGS. 27A to 27C illustrates a process of presetting a threshold value TH in the prior art photoelectric switch device 800. The prior art photoelectric switch 800 is provided with a threshold value trimming dial 810, a detection indicator lamp 811 and a detection stability indicator lamp 812. As shown in FIG. 27A, after putting an object 900 at a specified axial position in the optical axis Xp of the photoelectric switch device 800 and powering on the photoelectric switch device 800 to project light rays to the object 900, the threshold value trimming dial 810 is continuously turned clockwise until the detection indicator lamp 811 turns on. Then, the threshold value trimming dial 810 is set to a position indicated by a reference numeral “1”. Subsequently, as shown in FIG. 27B, after removing the object 900 from the optical axis Xp which turns off the detection indicator lamp 811 and directs the light rays to a reflective reference object 901, the threshold value trimming dial 810 is further continuously turned clockwise until the detection indicator lamp 811 turns on again. The threshold value trimming dial 810 is set to a position indicated by a reference numeral “3”. If there is no reflective reference object 901, the threshold value trimming dial 810 is turned clockwise until it reaches an extreme position. Finally, as shown in FIG. 27C, the threshold value trimming dial 810 is turned back counterclockwise to a midpoint, namely a position indicated by a reference numeral “2”, between the positions “1” and “3”. As a result, a specific axial position ST is set up at a position midway between the object 900 and the reference object 901. In other words, a threshold value TH meeting with the specific axial position ST is established. In this state, it is examined that both detection indicator lamp 811 and detection stability indicator lamp 812 turn on while there is an object 900 present in the field of detection and that, while the detection stability indicator lamp 812 turns on, the detection indicator lamp 811 turns off when the object 900 is removed from the field of detection. If the detection stability indicator lamp 811 does not turn on during the threshold value setting process this indicates that there is only a small margin between the position of the object 900 and the specific axial position ST and/or between the specific axial position ST and the position of the reference object 901. After resetting the object 900, the same threshold value setup process shown in FIGS. 27A to 27C is repeated.
In the prior art photoelectric switch 800, it is too troublesome for the user to turn the threshold value trimming dial 810 while observing both detection indicator lamp 811 and detection stability indicator lamp 811 in order to set and adjust a threshold value. As a result, it is difficult to precisely recognize the axial position ST and in consequence to achieve a fine adjustment in the threshold value. Further, it is difficult to judge the margin between the position of the object 900 and the specific axial position ST as well as the margin between the specific axial position ST and the position of the reference object 901.